MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur télécharger pdf (Page 5)

dominant bit state always wins out over a recessive bit state. In the recessive state, the

differential voltage CANH and CANL is less than the minimum threshold (i.e., less than

0.5V receiver input and less than 1.5V transmitter output). In the dominant state, the

differential voltage CANH and CANL is greater than the minimum threshold.

The ISO-11898 CAN bus specifies that a device on that bus must be able to drive a

forty-meter cable at 1Mb/s. A much longer bus length can usually be achieved by

lowering the bus speed. Figure 9.4 shows the variation of bus length with the

communication speed. For example, with a bus length of one thousand meters we can

have a maximum speed of 40Kb/s.

A CAN bus is terminated to minimize signal reflections on the bus. The ISO-11898

requires that the bus has a characteristic impedance of 120 ohms. The bus can be

terminated by one of the following methods:



Standard termination



Split termination



Biased split termination

In standard termination, the most common termination method, a 120-ohm resistor is

used at each end of the bus, as shown in Figure 9.5(a). In split termination, the ends

of the bus are split and a single 60-ohm resistor is used as shown in Figure 9.5(b).

Split termination allows for reduced emission, and this method is gaining popularity.

Biased split termination is similar to split termination except that a voltage divider

400

100 1000

760

1120

Speed (bps)

Bus length (m)

Figure 9.4: CAN bus speed and bus length

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MikroElektronika PIC Microcontrollers PIC18 Manuel d'utilisateur Page 5